The present invention relates generally to capacitive sensors used to determine the presence or level of an object, fluid or materials.
Capacitive sensors are used extensively for level measurement and proximity detection. A conventional capacitive sensor, which includes one or more conductive plates, is sensitive to changes in the dielectric constants of materials or fluids near or surrounding the plates. The capacitive sensor detects the presence or lack of material in the vicinity of the plates by measuring the capacitance between the plates, which is proportional to the dielectric constant of the material filling the space between the plates. By measuring this capacitance, the quantity of material (for level measurement) or the existence of the material (for proximity detection) may be determined. Similarly, another conventional form of capacitive sensor, which uses a sensing antenna, e.g., a long wire or strip immersed into a tank or storage bin holding a variable level of fluid or material, measures the level of the fluid or material by sensing and measuring the capacitance of the sensing antenna.
The accuracy of conventional capacitive sensors is based in large part on the dielectric constant of the material to be sensed. For example, when sensing capacitance, a fifty percent change in relative permittivity (the dielectric constant) causes a corresponding fifty percent change in the measured capacitance, i.e., the relationship is linear. Designing and producing capacitive sensors is therefor hampered by the sensitivity of conventional capacitive sensors to changes in the dielectric constant of the material to be sensed.
As an example, one application for a conventional capacitive sensor is a gasoline fuel gauge, which typically incorporates a sensor having a pair of parallel conductors that extend vertically into the fuel tank. As the fuel level rises, the capacitance measured across the two conductors increases because the dielectric constant of the fuel is higher than the air that it replaces. One problem that can arise in this convention application is when a small amount of water exists in the fuel tank together with the fuel. As the dielectric constant of water is significantly higher (about 80) than that of gasoline (about 2), even a small amount of water near the sensor can cause the capacitance to rise to a level corresponding to a full tank.
A similar situation may arise with a conventional capacitive sensor used as a proximity detector. A proximity detector typically compares a measured capacitance to a predetermined threshold and determines if materials are near the sensor by comparing the measured capacitance to the predetermined threshold. If the capacitance associated with the material can vary significantly (e.g., due to differing dielectric constants of the materials) it can be difficult to establish an appropriate threshold value.
There is a need, therefore, for a capacitive material sensor that is insensitive to changes or fluctuations in the dielectric constant of the fluid or material to be sensed. The preferred sensor would be capable of first extracting information regarding the nature of the sensed fluid or material and then capable of automatically compensating the sensor readings based on such information. Preferably, the sensor would accurately measure a variety of fluids or materials and be capable of manufacture at a relatively low cost.
The present invention overcomes these traditional difficulties and provides a capacitive sensor that automatically compensates for variations in its environment and operates reliably when used to measure or detect fluid or materials that may have varying dielectric constants. The capacitive level sensor incorporates multiple actual sensors (reference electrodes) that are used to extract information on the nature or composition of the fluid or material to be sensed and to allow the sensor to adjust or scale the level measurement based upon such extracted information. The sensor thus compensates for variations in the composition of the material or fluid being sensed.
The level sensor of the present invention includes a sensor array, which may take one of may forms, adapted to be disposed in the reservoir along an axis of measurement of the fluid or material. The sensor array includes a plurality of reference electrodes, wherein the capacitance of each of the electrodes varies in accordance both with the extent of the array""s immersion in the fluid or material and the dielectric constant of the fluid or material. The plurality of reference electrodes preferably includes a lower electrode adapted to be immersed within the fluid or material in the reservoir and whose capacitance provides an estimate of the dielectric constant of the fluid or material contained within the reservoir and a middle electrode whose capacitance varies from a calibrated initial value to a value that is dependent on the level of and the dielectric constant of the fluid or material contained within the reservoir as determined from the capacitance of the lower electrode. The level sensor further includes a circuit capable of measuring the current required to repetitively charge each reference electrode to a predetermined voltage and for converting each current to a digitized sensor voltage representing the capacitance of each electrode. A microprocessor then receives and processes the digitized sensor voltages to determine the fluid or material level such that the fluid or material level is determined from the capacitance of the middle electrode and is compensated by the amount and rate of change of the capacitance of the lower electrode.
The reference electrodes may, in certain embodiments, include an upper electrode adapted to be positioned above the fluid or material level that provides an estimate of the drift in the circuit, that is then used to further compensate the calculation of the fluid or material level. The reference electrodes may comprise a relatively large number of electrodes positioned along the length of the sensor array such that the particular upper, lower, and middle electrode used for the calculation of the level are dynamically selected. In another embodiment of the present invention, the sensor array may include a compensation electrode formed from an electrode positioned at each end of the sensor array, wherein the electrodes are electrically coupled together such that the sensor array may be disposed in the reservoir regardless of its orientation.